Process for forming a barrier layer on ceramic molds suitable for use for high temperature eutectic superalloy casting

Abstract

A method of forming a metal oxide barrier layer at the interface between a refractory oxide-silica investment mold and a contained metal superalloy includes the firing of the mold or the mold-metal system, in a controlled prevailing atmosphere having a predetermined amount of oxygen therein. The barrier layer enables directional solidification of the contained superalloy at elevated temperatures to occur without metal-mold reaction.

We claim as our invention: 
     
       1. A method for forming a barrier layer at the mold-metal interface in a mold suitable for the casting and directional solidification of superalloys therein including the process steps of: placing a mold made of a material comprising a first refractory oxide bonded together by silica within a furnace;   introducing a controlled prevailing atmosphere into the furnace, and   heating the mold in the controlled prevailing atmosphere at an elevated temperature for a sufficient period of time to at least dissolve some of the first refractory oxide into the silica.   
     
     
       2. The method of claim 1 wherein the material of the first refractory oxide is one selected from the group consisting of aluminum oxide, calcium oxide, yttrium oxide and magnesium oxide.   
     
     
       3. The method of claim 2 wherein the controlled prevailing atmosphere is a reducing atmosphere for the silica material of the mold.   
     
     
       4. The method of claim 2 wherein the controlled prevailing atmosphere is formed by introducing a predetermined amount of an oxygen-bearing gas into an inert carrier gas.   
     
     
       5. The method of claim 4 wherein the oxygen-bearing gas is pure oxygen.   
     
     
       6. The method of claim 4 wherein oxygen comprises from 0.01 to 5% of the gas mixture.   
     
     
       7. The method of claim 4 wherein the inert gas is argon.   
     
     
       8. The method of claim 2 wherein the material of the first refractory oxide is aluminum oxide.   
     
     
       9. The method of claim 8 wherein the material composition of the first refractory oxide is from about 80.0 to 99.9% by weight aluminum oxide and the remainder is silica.   
     
     
       10. The method of claim 1 wherein heating of the mold is at a sufficiently high temperature for a sufficient time to form a barrier layer comprising a second refractory oxide integral with the remaining material of the mold, the barrier layer having a surface defining in part the cavity within the mold into which a melt of metal is cast.   
     
     
       11. The method of claim 10 wherein the controlled prevailing atmosphere is a reducing atmosphere for the silica material of the mold.   
     
     
       12. The method of claim 10 wherein the first and second refractory oxides are the same.   
     
     
       13. The method of claim 12 wherein the refractory oxide is one selected from the group consisting of aluminum oxide, calcium oxide, yttrium oxide and magnesium oxide.   
     
     
       14. The method of claim 13 wherein the refractory oxide is aluminum oxide.   
     
     
       15. The method of claim 1 wherein firing of the mold is practiced prior to the casting of a melt of metal into the cavity of the mold.   
     
     
       16. The method of claim 15 wherein the controlled prevailing atmosphere is a reducing atmosphere for the silica material of the mold.   
     
     
       17. The method of claim 1 and including the additional process steps of: casting a melt of metal into a cavity in the mold,   forming a barrier layer of a second refractory oxide along a selected portion of the surfaces of the cavity of the mold and integral with the material of the mold, the barrier layer having a thickness to substantially prevent the molten metal to penetrate into the mold and to substantially retain the desired composition of the cast metal.   
     
     
       18. The method of claim 17 wherein the controlled prevailing atmosphere is a reducing atmosphere for the silica material of the mold.   
     
     
       19. The method of claim 18 wherein the supplied prevailing atmosphere comprises a mixture of an inert carrier gas and a predetermined amount of an oxygen-bearing gas.   
     
     
       20. The method of claim 19 wherein the supplied prevailing atmosphere is provided by introducing pure oxygen into the inert gas.   
     
     
       21. The method of claim 20 wherein oxygen comprises from 0.01 to 5% of the gas mixture.   
     
     
       22. The method of claim 21 wherein the inert gas is argon.   
     
     
       23. The method of claim 19 wherein the prevailing reducing atmosphere is provided by introducing water vapor into the inert carrier gas.   
     
     
       24. The method of claim 19 wherein the prevailing reducing atmosphere is provided by introducing air into the inert carrier gas by aspiration through an aperture in the walls of the furnace.   
     
     
       25. The method of claim 15 wherein the supplied prevailing atmosphere comprises a mixture of an inert carrier gas and a predetermined amount of an oxygen-bearing gas.   
     
     
       26. The method of claim 25 wherein the supplied prevailing atmosphere is provided by introducing pure oxygen into the inert carrier gas.   
     
     
       27. The method of claim 26 wherein oxygen comprises from 0.01 to 5% of the gas mixture.   
     
     
       28. The method of claim 27 wherein the inert gas is argon.   
     
     
       29. The method of claim 25 wherein the prevailing reducing atmosphere is provided by introducing water vapor into the inert carrier gas.   
     
     
       30. The method of claim 25 wherein the prevailing reducing atmosphere is provided by introducing air into the inert carrier gas by aspiration through an aperture in the walls of the furnace.   
     
     
       31. The method of claim 17 wherein the supplied prevailing atmosphere comprises a mixture of an inert carrier gas and a predetermined amount of an oxygen-bearing gas.   
     
     
       32. The method of claim 31 wherein the supplied prevailing atmosphere is provided by introducing pure oxygen into the inert carrier gas.   
     
     
       33. The method of claim 32 wherein oxygen comprises from 0.01 to 5% of the gas mixture.   
     
     
       34. The method of claim 33 wherein the inert gas is argon.   
     
     
       35. The method of claim 31 wherein the prevailing atmosphere is provided by introducing water vapor into the inert carrier gas.   
     
     
       36. The method of claim 31 wherein the prevailing atmosphere is provided by introducing air into the inert carrier gas by aspiration through an aperture in the walls of the furnace.   
     
     
       37. The method of claim 31 wherein the oxygen bearing gas is carbon monoxide.   
     
     
       38. The method of claim 31 including practicing the process step of heating the mold in the furnace with a graphite susceptor, and   forming carbon monoxide to mix in the inert carrier gas by reacting the graphite of the susceptor with the oxygen-bearing gas in the prevailing atmosphere introduced into the furnace.   
     
     
       39. The method of claim 31 including practicing the process step prior to heating the mold to an elevated temperature of disposing a body of carbon-bearing material within the confines of the furnace, and   practicing the process step subsequent to heating the mold to an elevated temperature,   forming carbon monoxide in the inert carrier gas by reacting the carbon-bearing material with the oxygen-bearing gas in the prevailing atmosphere introduced into the furnace.   
     
     
       40. The method of claim 19 wherein the oxygen-bearing gas is carbon monoxide.   
     
     
       41. The method of claim 31 wherein the prevailing atmosphere is a gas selected from the group consisting of hydrogen and hydrogen containing a predetermined amount of water vapor therein.   
     
     
       42. The method of claim 17 wherein the supplied prevailing controlled atmosphere is slightly oxidizing to the melt material.   
     
     
       43. The method of claim 31 wherein the inert carrier gas is argon.   
     
     
       44. The method of claim 31 wherein the supplied prevailing atmosphere comprises a mixture of an inert carrier gas and a predetermined amount of an oxygen-bearing gas.   
     
     
       45. The method of claim 44 wherein the inert carrier gas is argon.   
     
     
       46. The method of claim 44 wherein the supplied prevailing atmosphere is provided by introducing pure oxygen into the inert carrier gas.   
     
     
       47. The method of claim 46 wherein oxygen comprises from 0.01 to 5% of the gas mixture.   
     
     
       48. The method of claim 47 wherein the inert carrier gas is argon.   
     
     
       49. The method of claim 44 wherein the prevailing atmosphere is provided by introducing water vapor into the inert carrier gas.   
     
     
       50. The method of claim 44 wherein the prevailing atmosphere is provided by introducing air into the inert carrier gas by aspiration through an aperture in the walls of the furnace.   
     
     
       51. The method of claim 44 wherein the oxygen bearing gas is carbon monoxide.   
     
     
       52. The method of claim 44 including practicing the process steps of heating the mold in the furnace with a graphite susceptor, and   forming carbon monoxide to mix in the inert carrier gas by reacting the graphite of the susceptor with the oxygen-bearing gas in the prevailing atmosphere introduced into the furnace.   
     
     
       53. The method of claim 44 including practicing the process step, prior to heating the mold to an elevated temperature, of disposing a body of carbon-bearing material within the confines of the furnace, and   practicing the process step, subsequent to heating the mold to an elevated temperature, of   forming carbon monoxide in the inert carrier gas by reacting the carbon-bearing material with the oxygen-bearing gas in the prevailing atmosphere introduced into the furnace.   
     
     
       54. The method of claim 44 wherein the prevailing atmosphere is a gas selected from the group consisting of hydrogen and hydrogen containing a predetermined amount of water vapor therein.